Method and system for reducing magnetic field instabilities in a magnetic resonance system

ABSTRACT

A method, device, and system for reducing inhomogeneity in an imaging magnetic field during magnetic resonance imaging is described. The method includes generating a corrective magnetic field during imaging, the corrective magnetic field having a first magnetic field component and a second magnetic field component with a phase separation therebetween. The first and second components are generated according to a stability parameter decomposed from a stability field that correct an instability identified within the imaging magnetic field.

FIELD

The present disclosure is related to systems and methods for magneticresonance. More particularly, the disclosure relates to systems andmethods for reducing magnetic field instabilities that are constant orsporadic in a magnetic resonance system over time.

BACKGROUND

Magnetic resonance imaging (MRI) is generally performed with very strongstatic magnetic fields. The static magnetic field, referred to as the“main field” or “BO field”, is responsible for polarizing nuclei and isrequired for imaging during nuclear magnetic resonance.

A spatially homogeneous BO field is desired across the imagingfield-of-view (FOV) to ensure equal polarization of the subject and toremove BO-inhomogeneity weighting in the reconstructed images. MRInecessitates the use of multiple acquisitions that perform nuclearexcitation and relaxation, as the entire FOV is traversed by imaginggradients. Thus, any variation in the BO-field fromacquisition-to-acquisition over time tends to manifest itself asartifacts or ghosts in the reconstructed images. As such, temporalhomogeneity of the BO field is important.

Aside from static sources of magnetic field instabilities that do notchange over time, such as ferrous material located inside the imagingFOV, and paramagnetic patient implants etc., magnetic fieldinstabilities may arise from sources within, or in the vicinity of, theMRI scanner. Such sources of instability may be cyclical or sporadic.They include movement of a cold head in a superconducting magnet system,mechanical vibrations of the superconducting structure, presence ofmoving ferromagnetic material outside of the fringe field of the MRIscanner, and eddy current production from moving electrically-conductivematerial in the presence of a spatially-varying magnetic field.

These field instabilities may have a relative spatial distribution thatchanges as a function of time. Thus, there remains a need to providesystems and methods for reducing or otherwise eliminating magnetic fieldinstabilities caused by temporal instabilities in magnetic resonancesystems.

SUMMARY

In some examples, the present disclosure provides method for reducinginhomogeneity in an imaging magnetic field during magnetic resonanceimaging, the method comprising: generating a corrective magnetic fieldduring imaging, the corrective magnetic field having a first magneticfield component and a second magnetic field component with a phaseseparation therebetween, the first and second components being generatedaccording to a stability parameter decomposed from a stability fieldthat corrects an instability identified within the imaging magneticfield.

In some examples, the present disclosure provides an electromagnetassembly for reducing inhomogeneity in an imaging magnetic field in animaging bore of a magnetic resonance imaging system during imaging, theelectromagnet assembly configured to generate a corrective magneticfield according a stability parameter that corrects an instabilityidentified within the imaging magnetic field, the corrective magneticfield having a first magnetic field component and a second magneticfield component with a phase separation therebetween.

In some examples, the present disclosure provides a system for reducinginhomogeneity in an imaging magnetic field in an imaging bore of amagnetic resonance imaging system during imaging, the system comprising:an electromagnet assembly configured to generate a corrective magneticfield in the imaging bore, the corrective magnetic field being generatedaccording to a stability parameter that corrects an instabilityidentified within the imaging magnetic field, the corrective magneticfield having a first magnetic field component and a second magneticfield component with a phase separation therebetween.

The foregoing and other aspects and advantages of the present disclosurewill appear from the following description. In the description,reference is made to the accompanying drawings that form a part hereof,and in which there is shown by way of illustration a preferredembodiment. Such embodiment does not necessarily represent the fullscope of the disclosure, however, and reference is made therefore to theclaims and herein for interpreting the scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example, to the accompanyingdrawings which show example embodiments of the present application, andin which:

FIG. 1 is a flowchart setting forth the steps of an example method forreducing inhomogeneity in an imaging magnetic field during magneticresonance imaging;

FIG. 2 is a flowchart setting forth the steps of an example method forreducing sporadic inhomogeneity in an imaging magnetic field duringmagnetic resonance imaging;

FIG. 3 depicts shim coils according an example embodiment of the presentdisclosure for reducing inhomogeneity in an imaging magnetic fieldduring magnetic resonance imaging;

FIG. 4 illustrates a frequency spectrum prior to compensation of fieldinstabilities;

FIG. 5 illustrates a frequency spectrum after compensation of fieldinstabilities, performed in accordance with examples described herein;

FIG. 6 illustrates the effect of actively tuning the current source tothat of the pulse tube and directly cancelling both the real andimaginary (phase and magnitude) of the field instability; and

FIG. 7 is a block diagram of an example magnetic resonance imaging(“MRI”) system, which incorporate the shim coils of FIG. 3.

DETAILED DESCRIPTION

Described herein are systems, devices, and methods for mitigating theeffects of temporal magnetic field instabilities caused by cyclical orsporadic perturbations over time in the main magnetic field of magneticresonance systems, such as magnetic resonance imaging (“MRI”) systems,nuclear magnetic resonance (“NMR”) systems, or the like.

Referring to FIGS. 1 and 2, there are illustrated example methods formitigating temporal magnetic field instability during MRI imaging.

Method 100 shown in FIG. 1 sets forth example steps for reducinginhomogeneity in an imaging magnetic field during magnetic resonanceimaging. At 102, an instability in the magnetic field is identified. Theinstability has a relative spatial distribution that changes over time.In some examples, the instability may have a spatial distribution thatchanges over time in a cyclical manner. For example, the instability maybe due to vibrations caused by operation of a cryocooler, movement offerrous materials in the fringe field of the MRI and/or the induction ofextraneous eddy currents on conducting structures in the MRI. Evenharmonic sources of BO instability can produce eddy currents that resultin a non-uniform Bz distortion across the field-of-view (FOV) and accruespatially-variant phase.

In certain applications, after the magnetic field in an imaging regionhas been mapped at multiple time points (to get a time-space map), themagnetic field in the imaging region is processed. In certainapplications, the magnetic field in the imaging region is thendecomposed into two or more general components. The instability may bethen identified as two or more unwanted general components in theinhomogeneous field.

At 104, a stability field is then determined in response to theinstability in the imaging magnetic field. The stability field is amagnetic field that is similar but opposite to that caused by theinstability in the imaging magnetic field. In other words, the stabilityfield may be a magnetic field that has similar, or the same, spatialdistribution as the unwanted component, but is opposite thereto.

At 105, this stability field may be decomposed into a spatial (e.g.spherical harmonics) and/or a temporal (e.g. Fourier components) basis.In one embodiment, the stability field is first decomposed temporally(into the Fourier or frequency domain). Next, the magnitude/amplitudeand phase of a given Fourier component is extracted for each spatialposition as stability parameters. For example, the most dominant,common, frequency between all points is extracted from the spectrum.

The amplitude and phase of the selected Fourier component for eachspatial position (i.e. the stability parameters) are then used at 106 tocreate at least two target spatial magnetic field components with aphase separation therebetween, such as a first magnetic field and asecond magnetic field. The first magnetic field component may be“in-phase” (e.g., by sampling the magnetic field with the phase at eachspatial location to be 0 degrees) and the second magnetic fieldcomponent may be “out-of-phase” (e.g., by sampling the magnetic fieldwith the phase at each spatial location to be shifted 90 degrees, thusout-of-phase relative to the in-phase component).

At 107, two electromagnets are then designed and created to produce therespective first and second, or in-phase and out-of-phase, targetmagnetic fields. The in-phase (sometimes denoted “real”) andout-of-phase (sometimes denoted “imaginary”) electromagnets would thenbe driven with a first waveform and a second waveform respectively.

A corrective magnetic field, which comprises the in-phase magnetic fieldcomponent and the out-of-phase magnetic field component (or first andsecond component), is then generated in the imaging region at 108 toeliminate the Bz-component of the field instability. In other words, thefirst and second components are being generated according to stabilityparameters that were decomposed from the stability field which correctsan instability identified within the imaging magnetic field. Asunderstood by the skilled person, the in-phase and out-of-phase magneticfield components generated by the first and second waveforms aregenerated with different phases, or not in phase with each other.

In certain applications, the phase shift or separation between the firstand second waveforms (or between the first and second components) may befixed, for example, where the fixed phase shift may be 90 degrees. Whenthe stability field is decomposed into its real and imaginarycomponents, the phase shift between the first and second waveforms maybe 90 degrees. In other applications, the phase shift between the firstand second waveforms may be smaller or larger than 90 degrees.

To generate the corrective magnetic field, a first electromagnet and aseparate second electromagnet in the MRI system may be driven at 110 and112. In particular, as noted above, the first electromagnet would bedesigned to generate the first magnetic field of the corrective magneticfield according to the first waveform and the second electromagnet wouldbe designed to generate the second magnetic field of the correctivemagnetic field according to the second waveform. The first electromagnetmay, thus, be a first shim coil and the second electromagnet may be asecond shim coil.

In this manner, the temporal BO inhomogeneity is accounted for when the“imaginary” second shim coil is driven out of phase, 90 degrees out ofphase for example, with the “real” first shim coil.

Each coil is responsible for removing the respective variation in themagnetic field. When these two coils are driven simultaneously with afixed phase shift between them, and with the appropriate amount ofcurrent (so as to produce an equal but opposite magnetic field to thatof the instability), a spatially-varying distribution in response to theBO instability, or the corrective magnetic field, is generated.

By superimposing or merging the corrective magnetic field with the mainmagnetic field together, the magnetic field instability or inhomogeneityis reduced, neutralized, or cancelled spatially and temporally.

While method 100 has been described as including identifying theinstability (102), determining the stability field (104), decomposingthe stability field into stability parameters (105), creating the firstand second magnetic field components (106), and designing the first andsecond electromagnets (107), steps 102, 104, 105, 106, and 107 areoptional. Only 108, that of generating the corrective magnetic fieldaccording to the at least two spatial magnetic field components (such asthe first and second magnetic field components) with a phase separationtherebetween is required for the instability to be mitigated orcorrected. In that manner, steps 102, 104, 105, 106, and 107 may beperformed separately from 108, such as during calibration, installation,etc. prior to step 108.

As well, that the corrective magnetic field is generated by drivingfirst and second shims according to the first and second waveforms withdifferent phases, to generate the first and second magnetic fieldcomponents, 110 and 112, is also optional. The corrective magnetic fieldmay be generated by a different electromagnet system or structure solong as the first and second magnetic field components of the correctivemagnetic field with a phase separation therebetween is generated.

While two magnetic field components are discussed above, three or moremagnetic field components may be generated with phase separationstherebetween (i.e., each component is phase-shifted relative to eachother component, such that none of the components are at the samephase). If there are three (or more) magnetic field components, any oneof those might be considered the “in-phase” component, while the othertwo (or more) may be considered the “out-of-phase” components. As well,if three (or more) spatial magnetic field components are generated,three (or more) electromagnets may then be designed and created toproduce the respective first, second, and third (or more) targetmagnetic fields.

If the instability is predictable or cyclical (e.g., arising due to apredictable or cyclical source), method 100 may be performed in a closedloop fashion, where the BO fluctuation may be measured, characterized,and reduced or nulled without feedback in space and time (i.e. magnitudeand phase).

If the instability is unpredictable or sporadic, method 200 may beperformed in an open loop manner, where the magnetic field instabilitymay be measured, instability feedback acquired, and the correctivemagnetic field changed according to the feedback.

Method 200 shown in FIG. 2 sets forth example steps for reducingsporadic inhomogeneity in an imaging magnetic field during magneticresonance imaging. Method 200 may involve steps 102 to 112 of method100. In particular, method 200 includes at least generating thecorrective magnetic field in the imaging region at 108. The correctivemagnetic field, comprising the first and second magnetic fieldcomponents, is generated according to the first waveform (110) and thesecond waveform (112). As noted above, the first and second waveformsare generated out of phase with one another.

At 202, a change in the instability in the main imaging magnetic fieldmay be detected. The change may be detected by a system of fielddetectors built into the MRI system. Examples of such field detectorsinclude field sensors inside the bore of the magnet and/or field sensorsoutside the bore of the magnet in the MRI room that are monitoredcontinuously. The system may include field measuring devices locatedwithin or around the system. For example, the system may also involve asensor on an external device known to be involved in the source of theinstability, for example, a pressure sensor on a cryocooler gas line tomeasure the cycle, or a sensor on an elevator to monitor its level.

This detected change may be unpredictable and sporadic, such as thepresence of moving ferromagnetic material in the vicinity of the fringefield of the MRI scanner. This could be due to the presence of movingcars, elevators, or subway lines, etc. in the vicinity of the MRIscanner.

After the change is detected, the parameters at which the first andsecond electromagnets are driven to reduce the instability in theimaging magnetic field due to the change may be changed at 204. Thedriving parameters of the electromagnets can be fixed or dynamically setbased on the number of compensation coils deployed in the system and therequired compensation accuracy. The first and second compensation coilsmay be dynamically shifted to cancel impingent electromagnetic fields.For example, the phase between the two waveforms, and the relativeamplitude of the waveforms may be shifted.

FIG. 3 illustrates an example of an exploded view of electromagneticassembly 30 comprising actively driven electromagnets 32, 34 that may beused in methods 100 and 200 for reducing inhomogeneity in the imagingmagnetic field during magnetic resonance imaging. The coils inelectromagnet 32 depicted on the left are specifically designed togenerate the first portion of the corrective magnetic field according tothe first waveform, which in this example is the real component of thecorrective magnetic field. The coils in electromagnet 34 depicted on theright are specifically designed to generate the second portion of thecorrective magnetic field according to the second waveform, which inthis example is the imaginary component of the corrective magneticfield. Electromagnets 32, 34 may be driven jointly or independently.

As noted above, first and second electromagnets 32, 34 in the depictedembodiment are shim coils. They may be used simultaneously, out of phasewith one another, to reduce spatial and/or temporal inhomogeneity in theimaging magnetic field.

Custom electromagnets for use in MRI are typically referred to as“shims”. Shims refer to any component in the MRI scanner—active orpassive—that provides a favourable adjustment of the main fieldhomogeneity. The shims described here are driven from a harmonic tonegenerator that provides both adequate current and a phase shift to‘tune’ the electromagnets to the instability. If required, severalhigher-ordered harmonics can be included to reduce field instability.

FIGS. 4 and 5 demonstrate an example of the above compensation effectwhen using first and second electromagnets 32, 34 when the instabilityis caused by a pulse tube in a cold head motor. A cold head motor expelsa pulse of air that causes vibration every 1.4 Hz. FIG. 4 illustratesthe frequency spectrum prior to compensation of the instability. Thex-axis is in Hz, and the problem or instability is shown atapproximately +/−1.4 Hz.

FIG. 5 illustrates the frequency spectrum after active compensation ofthe instability has been performed, for example as described above. Asreadily seen, the instability or problem at +/−1.4 Hz has beensubstantially mitigated after active compensation using first and secondelectromagnets 32, 34. Sensors on the cold head motor can trigger thecompensation, for example, by detecting the change in the instability at202 as described in method 200, thus dynamically syncing up thecompensation with the cold head motor.

FIG. 6 is a cost function graph that demonstrates the effect of activelytuning the current source to that of the pulse tube and directlycancelling both the real and imaginary (phase and magnitude) of thefield instability. A lower cost is better or preferable in each case. Assuch, the optimal phase for real coil 32 is around 90 degrees, whereasthe optimal phase for imaginary coil 34 is around 0 or 180 degrees. Asunderstood by the skilled person, there are different optimal phases fordifferent components. As such, the coils are to be driven with differentphases (or with a phase shift therebetween) and amplitudes in order tocompensate for magnetic field instabilities in space and time.

While electromagnetic assembly 30 is described to have twoelectromagnets 32, 34, it would be understood that more than twoelectromagnets or shims may be used. For example, another pair of shimsor electromagnets, as described above, may be added for each problemsource or instability.

In some examples, two shims may be designed based on the overallmagnetic field distribution. Such a magnetic field distribution would,understandably, be specific to the particular location of the MRIsystem. In yet a further alternate embodiment, the corrective magneticfield may be decomposed into three or more parts or components, eachwith differing phases. In such a case, three or more shim coils may bedesigned, one shim to correspond with each decomposed component.

Referring now to FIG. 7, an example of a MRI system 700 is illustratedwhich incorporates electromagnetic assembly 30 described above. In someexamples, MRI system 700 may at least in part be used to perform methods100 and 200. Additionally, the following discussion of MRI system 700leads to further understanding of methods 100 and 200. However, it is tobe understood that MRI system 700, and methods 100 and 200 can bevaried, and need not work exactly as discussed herein in conjunctionwith each other, and that such variations are within scope of theappended claims.

MRI system 700 includes an operator workstation 702, which willtypically include a display 704; one or more input devices 706, such asa keyboard and mouse; and a processor 708. The processor 708 may includea commercially available programmable machine running a commerciallyavailable operating system. The operator workstation 702 provides theoperator interface that enables scan prescriptions to be entered intothe MRI system 700.

In general, the operator workstation 702 may be coupled to four servers:a pulse sequence server 710; a data acquisition server 712; a dataprocessing server 714; and a data store server 716. The operatorworkstation 702 and each server 710, 712, 714, and 716 are connected tocommunicate with each other. For example, the servers 710, 712, 714, and716 may be connected via a communication system 740, which may includeany suitable network connection, whether wired, wireless, or acombination of both. As an example, the communication system 740 mayinclude both proprietary or dedicated networks, as well as opennetworks, such as the internet

The pulse sequence server 710 functions in response to instructionsdownloaded from the operator workstation 702 to operate a gradientsystem 718 and a radiofrequency (“RF”) system 720. Gradient waveformsnecessary to perform the prescribed scan are produced and applied to thegradient system 718, which excites gradient coils in an assembly 722 toproduce the magnetic field gradients Gx, Gy, and Gz used for positionencoding magnetic resonance signals. The gradient coil assembly 722forms part of a magnet assembly 724 that includes a polarizing magnet726 and a whole-body RF coil 728.

RF waveforms are applied by the RF system 720 to the RF coil 728, or aseparate local coil (not shown in FIG. 7), in order to perform theprescribed magnetic resonance pulse sequence. Responsive magneticresonance signals detected by the RF coil 728, or a separate local coil(not shown in FIG. 7), are received by the RF system 720, where they areamplified, demodulated, filtered, and digitized under direction ofcommands produced by the pulse sequence server 710. The RF system 720includes an RF transmitter for producing a wide variety of RF pulsesused in MRI pulse sequences. The RF transmitter is responsive to thescan prescription and direction from the pulse sequence server 710 toproduce RF pulses of the desired frequency, phase, and pulse amplitudewaveform. The generated RF pulses may be applied to the whole-body RFcoil 728 or to one or more local coils or coil arrays (not shown in FIG.7).

The RF system 720 also includes one or more RF receiver channels.

Each RF receiver channel includes an RF preamplifier that amplifies themagnetic resonance signal received by the coil 728 to which it isconnected, and a detector that detects and digitizes the I andquadrature components of the received magnetic resonance signal. Themagnitude of the received magnetic resonance signal may, therefore, bedetermined at any sampled point by the square root of the sum of thesquares of the and Q components:

M=√{square root over (I ² +Q ²)}

and the phase of the received magnetic resonance signal may also bedetermined according to the following relationship:

$\phi = {\tan^{- 1}\left( \frac{Q}{I} \right)}$

The pulse sequence server 710 also optionally receives patient data froma physiological acquisition controller 730. By way of example, thephysiological acquisition controller 730 may receive signals from anumber of different sensors connected to the patient, such aselectrocardiograph (“ECG”) signals from electrodes, or respiratorysignals from a respiratory bellows or other respiratory monitoringdevice. Such signals are typically used by the pulse sequence server 710to synchronize, or “gate,” the performance of the scan with thesubject's heart beat or respiration.

The pulse sequence server 710 also connects to a scan room interfacecircuit 732 that receives signals from various sensors associated withthe condition of the patient and the magnet system. It is also throughthe scan room interface circuit 732 that a patient positioning system734 receives commands to move the patient to desired positions duringthe scan.

The digitized magnetic resonance signal samples produced by the RFsystem 720 are received by the data acquisition server 712. The dataacquisition server 712 operates in response to instructions downloadedfrom the operator workstation 702 to receive the real-time magneticresonance data and provide buffer storage, such that no data is lost bydata overrun. In some scans, the data acquisition server 712 does littlemore than pass the acquired magnetic resonance data to the dataprocessor server 714. However, in scans that require information derivedfrom acquired magnetic resonance data to control the further performanceof the scan, the data acquisition server 712 is programmed to producesuch information and convey it to the pulse sequence server 710.

For example, during pre-scans, magnetic resonance data is acquired andused to calibrate the pulse sequence performed by the pulse sequenceserver 710. As another example, navigator signals may be acquired andused to adjust the operating parameters of the RF system 720 or thegradient system 718, or to control the view order in which k-space issampled. In still another example, the data acquisition server 712 mayalso be employed to process magnetic resonance signals used to detectthe arrival of a contrast agent in a magnetic resonance angiography(“MRA”) scan. By way of example, the data acquisition server 712acquires magnetic resonance data and processes it in real-time toproduce information that is used to control the scan.

The data processing server 714 receives magnetic resonance data from thedata acquisition server 712 and processes it in accordance withinstructions downloaded from the operator workstation 702. Suchprocessing may, for example, include one or more of the following:reconstructing two-dimensional or three-dimensional images by performinga Fourier transformation of raw k-space data; performing other imagereconstruction algorithms, such as iterative or back projectionreconstruction algorithms; applying filters to raw k-space data or toreconstructed images; generating functional magnetic resonance images;calculating motion or flow images; and so on.

Images reconstructed by the data processing server 714 are conveyed backto the operator workstation 702 where they are stored. Real-time imagesare stored in a data base memory cache (not shown in FIG. 7), from whichthey may be output to operator display 702 or a display 736 that islocated near the magnet assembly 724 for use by attending physicians.Batch mode images or selected real time images are stored in a hostdatabase on disc storage 738. When such images have been reconstructedand transferred to storage, the data processing server 714 notifies thedata store server 716 on the operator workstation 702. The operatorworkstation 702 may be used by an operator to archive the images,produce films, or send the images via a network to other facilities.

The MRI system 700 may also include one or more networked workstations742. By way of example, a networked workstation 742 may include adisplay 744; one or more input devices 746, such as a keyboard andmouse; and a processor 748. The networked workstation 742 may be locatedwithin the same facility as the operator workstation 702, or in adifferent facility, such as a different healthcare institution orclinic.

The networked workstation 742, whether within the same facility or in adifferent facility as the operator workstation 702, may gain remoteaccess to the data processing server 714 or data store server 716 viathe communication system 740. Accordingly, multiple networkedworkstations 742 may have access to the data processing server 714 andthe data store server 716. In this manner, magnetic resonance data,reconstructed images, or other data may be exchanged between the dataprocessing server 714 or the data store server 716 and the networkedworkstations 742, such that the data or images may be remotely processedby a networked workstation 742. This data may be exchanged in anysuitable format, such as in accordance with the transmission controlprotocol (“TCP”), the internet protocol (“IP”), or other known orsuitable protocols.

Electromagnetic assembly 30 may be incorporated into MRI system 700 in anumber of different ways. For example, in the embodiment depicted inFIG. 7, electromagnetic assembly 30 is positioned proximate a cold head12 of a mechanical cryocooler, cold head 12 being the source of themagnetic field instability in this case. Electromagnetic assembly 30 maybe driven by one of the controllers according to the methods describedabove.

Because the magnetic moment of the material depends on the magnitude ofthe external field, if the magnetic moments of the materials in the coldhead are reduced, the field instability effects will be reduced as well.The magnetic moment in the cold head materials can be reduced byshielding the magnetic field around the cold head with electromagnetassembly 30. Electromagnetic assembly 30 may be driven by one of thecontrollers according to the methods described above.

In an alternate embodiment, rather than being positioned proximate thesource of the magnetic field instability, electromagnetic assembly 30may form part of magnet assembly 724 and/or part of gradient coilassembly 722.

MRI system 700 may further include a sensor for detecting the change inthe instability in the imaging magnetic field. In such a case, thesensor would be operatively coupled to first and second electromagnetsor shim coils 32, 34 of electromagnetic assembly 30 for dynamicallychanging parameters at which first and second shim coils 32, 34 aredriven to reduce the instability in the imaging magnetic field due tothe change. As noted above, first and second electromagnets 32, 34 maybe driven in MRI system 700 together or independently.

It will be appreciated that the above magnetic field compensationmethods and systems may be implemented alone or in combination withother compensation methods and systems, including passive shimming,depending on the type and severity of the field instability.

While some embodiments or aspects of the present disclosure may beimplemented in fully functioning computers and computer systems, otherembodiments or aspects may be capable of being distributed as acomputing product in a variety of forms and may be capable of beingapplied regardless of the particular type of machine or computerreadable media used to actually effect the distribution.

At least some aspects disclosed may be embodied, at least in part, insoftware. That is, some disclosed techniques and methods may be carriedout in a computer system or other data processing system in response toits processor, such as a microprocessor, executing sequences ofinstructions contained in a memory, such as read-only memory (ROM),volatile random access memory (RAM), non-volatile memory, cache or aremote storage device.

A computer readable storage medium may be used to store software anddata which when executed by a data processing system causes the systemto perform various methods or techniques of the present disclosure. Theexecutable software and data may be stored in various places includingfor example ROM, volatile RAM, non-volatile memory and/or cache.Portions of this software and/or data may be stored in any one of thesestorage devices.

Examples of computer-readable storage media may include, but are notlimited to, recordable and non-recordable type media such as volatileand non-volatile memory devices, ROM, RAM, flash memory devices, floppyand other removable disks, magnetic disk storage media, optical storagemedia (e.g., compact discs (CDs), digital versatile disks (DVDs), etc.),among others. The instructions can be embodied in digital and analogcommunication links for electrical, optical, acoustical or other formsof propagated signals, such as carrier waves, infrared signals, digitalsignals, and the like. The storage medium may be the internet cloud, ora computer readable storage medium such as a disc.

Furthermore, at least some of the methods described herein may becapable of being distributed in a computer program product comprising acomputer readable medium that bears computer usable instructions forexecution by one or more processors, to perform aspects of the methodsdescribed. The medium may be provided in various forms such as, but notlimited to, one or more diskettes, compact disks, tapes, chips, USBkeys, external hard drives, wire-line transmissions, satellitetransmissions, internet transmissions or downloads, magnetic andelectronic storage media, digital and analog signals, and the like. Thecomputer useable instructions may also be in various forms, includingcompiled and non-compiled code.

At least some of the elements of the systems described herein may beimplemented by software, or a combination of software and hardware.Elements of the system that are implemented via software may be writtenin a high-level procedural language such as object oriented programmingor a scripting language. Accordingly, the program code may be written inC, C++, J++, or any other suitable programming language and may comprisemodules or classes, as is known to those skilled in object orientedprogramming. At least some of the elements of the system that areimplemented via software may be written in assembly language, machinelanguage or firmware as needed. In either case, the program code can bestored on storage media or on a computer readable medium that isreadable by a general or special purpose programmable computing devicehaving a processor, an operating system and the associated hardware andsoftware that is necessary to implement the functionality of at leastone of the embodiments described herein. The program code, when read bythe computing device, configures the computing device to operate in anew, specific and predefined manner in order to perform at least one ofthe methods described herein.

While the teachings described herein are in conjunction with variousembodiments for illustrative purposes, it is not intended that theteachings be limited to such embodiments. On the contrary, the teachingsdescribed and illustrated herein encompass various alternatives,modifications, and equivalents, without departing from the describedembodiments, the general scope of which is defined in the appendedclaims. Except to the extent necessary or inherent in the processesthemselves, no particular order to steps or stages of methods orprocesses described in this disclosure is intended or implied. In manycases the order of process steps may be varied without changing thepurpose, effect, or import of the methods described.

1. A method for reducing inhomogeneity in an imaging magnetic field during magnetic resonance imaging, the method comprising: generating a corrective magnetic field during imaging, the corrective magnetic field having a first magnetic field component and a second magnetic field component with a phase separation therebetween, the first and second components being generated according to a stability parameter decomposed from a stability field that corrects an instability identified within the imaging magnetic field.
 2. The method of claim 1, wherein the first and second magnetic field components are, respectively, an in-phase magnetic field component and an out-of-phase magnetic field component.
 3. The method of claim 2, wherein the phase separation is a 90 degrees.
 4. The method of claim 1, further comprising: identifying the instability within the imaging magnetic field; determining the stability field in response to the instability in the imaging magnetic field; decomposing the stability field into stability parameters, and creating the first and second magnetic field components from the stability parameters.
 5. The method of claim 1, further comprising designing a first electromagnet to generate the first magnetic field component, and designing a second electromagnet to generate the second magnetic field component.
 6. The method of claim 1, wherein generating the corrective magnetic field comprises: driving a first electromagnet designed to generate the first component of the corrective magnetic field with a first waveform; and driving a second electromagnet designed to generate the second component of the corrective magnetic field with a second waveform;
 7. The method of claim 6, wherein the first electromagnet is a first shim coil and the second electromagnet is a second shim coil.
 8. The method of claim 6, wherein the instability in the imaging magnetic field is constant or sporadic over time.
 9. The method of claim 8, wherein the instability is sporadic over time, the method further comprising: detecting a change in the instability in the imaging magnetic field; dynamically changing parameters at which the first and second electromagnets are driven to reduce the instability in the imaging magnetic field due to the change.
 10. An electromagnet assembly for reducing inhomogeneity in an imaging magnetic field in an imaging bore of a magnetic resonance imaging system during imaging, the electromagnet assembly configured to generate a corrective magnetic field according a stability parameter that corrects an instability identified within the imaging magnetic field, the corrective magnetic field having a first magnetic field component and a second magnetic field component with a phase separation therebetween.
 11. The electromagnet assembly of claim 10 comprising: a first electromagnet designed to generate the first component of the corrective magnetic field according to the first waveform; and a second electromagnet designed to generate the second component of the corrective magnetic field with a second waveform.
 12. The electromagnet assembly of claim 11, wherein the first electromagnet is a first shim coil, and the second electromagnet is a second shim coil.
 13. A system for reducing inhomogeneity in an imaging magnetic field in an imaging bore of a magnetic resonance imaging system during imaging, the system comprising: an electromagnet assembly configured to generate a corrective magnetic field in the imaging bore, the corrective magnetic field being generated according to a stability parameter that corrects an instability identified within the imaging magnetic field, the corrective magnetic field having a first magnetic field component and a second magnetic field component with a phase separation therebetween.
 14. The system of claim 13, wherein the first and second magnetic field components are, respectively, an in-phase magnetic field component and an out-of-phase magnetic field component.
 15. The system of claim 14, wherein the phase shift is a 90 degree phase shift.
 16. The system of claim 13, further comprising: a processor configured to: identify the instability within the imaging magnetic field; determine the stability parameter in response to the instability in the imaging magnetic field; and decompose the stability parameter into the first and the second components.
 17. The system of claim 13, wherein the electromagnet assembly comprises a first electromagnet designed to generate the first component of the corrective magnetic field with a first waveform; and a second electromagnet designed to generate the second component of the corrective magnetic field with a second waveform.
 18. The system of claim 13, wherein the first electromagnet is a first shim coil and the second electromagnet is a second shim coil.
 19. The system of claim 18, wherein the first and second shim coils are independently driven.
 20. The system of claim 17, further comprising: a sensor for detecting a change in the instability in the imaging magnetic field, the sensor operatively coupled to the first and second electromagnets for dynamically changing parameters at which the first and second shim coils are driven to reduce the instability in the imaging magnetic field due to the change. 